The present invention relates generally to printing plates mounted on cylindrical rollers in a printing press, and more particularly, is directed to an apparatus and method for laser ablating of printing plates and/or printing rollers to decrease variations in TIR and/or TAPER thereof in order to ensure cylindricity thereof.
It is well-known to use printing plates mounted on printing rollers in a printing operation. The printing rollers can include one or more sleeves and/or other layers thereon. The printing plates are etched or engraved in accordance with a desired image in order to effect the printing operation. It is also known to provide such etching or engraving of the image on the printing plate by means of a laser.
However, a problem with such known arrangements is that the raised surface of the printing plate, when mounted on the printing roller, is not adequately cylindrical. In other words, apart from the etched portions, there are variations in the radii at different positions of the raised surface of the printing plate, measured from the center of the printing roller to the raised surface of the printing plate, thereby providing variations in the cylindricity thereof.
These variations in cylindricity affect the print quality. Specifically, it is important that a good impression be provided to obtain a high grade print during a fast flexographic printing operation. However, tolerances in the printing roller and/or the printing plate cause variations in the cylindricity. In other words, these tolerances result in a lack of adequate cylindricity. In order to compensate for the same during a printing operation, it is necessary to provide greater pressure or impression of the printing roller and printing plate during a printing operation. However, in regions of the printing roller and printing plate having a greater radius than other regions, this results in various problems, such as general squeegeeing of the image, printing plate speed mismatch and a resultant general lower quality print.
It is therefore desirable that the outer surface of the printing plate mounted on the printing roller have a cylindricity without these high and low spots.
In this regard, there are two measurements that determine the cylindrical nature or cylindricity of the printing roller and/or printing plate. The first measurement is TIR (Total Indicated Run-out), which determines the circular nature of the cylindrical printing plate at one point during a single revolution of the printing plate. The second measurement is TAPER, which determines if there are any variations in the radius from end to end of the printing roller and/or printing plate, that is, in the lengthwise direction thereof.
Conventionally, in order to obtain a cylindrical outer surface without variations in TIR and/or TAPER, a grinding wheel has been used to mechanically grind the outer surface of the printing roller and/or printing plate to make the outer surface cylindrical. However, this is burdensome, costly and time consuming.